A vital organ, the kidneys serve several purposes including homeostatic functions such as regulating extracellular fluid volume, maintaining acid-base and electrolyte balance, and are essential regarding the secretion of metabolic waste. The kidneys play an important role in uric acid secretion/re-absorption. Kidney transporter protein abnormalities (i.e. altered function) are associated with various diseases, such as gout. In humans, purine nucleotides, nucleosides, and bases (i.e. adenine, inosine, and guanine) are metabolically degraded to urate (uric acid 1, FIG. 1) via xanthine.
Many organisms possess the enzyme uricase and metabolize 1 to the more water soluble allantoin 2, however this enzyme is absent in humans (Zubay, 1998; Neogi, 2011). A known antioxidant in the blood, elevated levels of uric acid (a condition known as hyperuricemia) can precipitate gout; a medical condition associated with repeated episodes of acute inflammatory arthritis caused by elevated urate serum level (Shin et al, 2011). Lifestyle and diet are also well known contributors to elevated serum urate (Sang et al, 2006). Renal function declines as we age to produce lower urate excretion. Furthermore, high uric acid levels in the body are believed to play a pivotal role in other diseases such as hypertension, insulin resistance, diabetes, chronic renal disease, diabetic renal disease, and cardiovascular disease (Sang et al, 2006; Feig et al, 2008; Anzai et al, 2010). Therefore, developing novel drugs that influence uric acid serum level are therapeutically important.
There are different drug strategies to control urate levels (FIG. 2). A few commercially available small molecule drugs are administered to lower serum urate in the United States. Allopurinol 3 is a xanthine oxidase inhibitor and the most commonly used urate-lowering drug in the US. Compound 3 occasionally causes Stevens Johnson syndrome and may be fatal (Lemke and Williams. 2008). Another drug, febuxostat 4 is as a non-purine xanthine oxidase inhibitor but known to afford cardiovascular complications. Probenecid 5, sulfinpyrazone 6 and benzbromarone 7 are uricosuric drugs; they increase uric acid renal excretion by inhibiting urate re-absorption via one or more transporter proteins (Shin et al, 2011).
Renal elimination plays a primary role in controlling uric acid serum level (Becker and Jolly, 2006; Anzai et al, 2007; Kutzing and Firestein, 2008). Urate is reabsorbed and secreted along the nephron. In humans, the apical surface (FIG. 3) contains human uric acid transporter 1 (hURAT1; SLC22A12), human organic anion transporter 10 (hOAT10; URAT2), and the natrium-dependent phosphate transporter 4, also called voltage-dependent human organic anion transporter 1 (NPT4, hOATv1; SLC17A3) (Shin et al, 2011; Enomoto et al, 2002; Roch-Ramel and Guisan, 1999; Mount et al, 2006). Consequently, these transporters interact with the urine.
The basolateral surface also contains transporters, but they are in contact with the blood and include: i) the facilitative glucose transporter 9, also known as the voltage-dependent uric acid transporter 1 (GLUT9, URATv1; SLC2A9) (Mount et al, 2006; Jutabha et al, 2010; Anzai et al, 2008; Caulfied et al, 2008) and human organic anion transporter proteins 1 (hOAT1; SLC22A6) and 3 (hOAT3; SLC22A8) known to have broad substrate specificity and transport, in addition to urate, they are known to transport NSAIDs, (3-lactams, and p-aminohippuric acid (Sekine et al, 1997).
Benzbromarone 7 effectively reduces serum urate levels and has been administered clinically in Japan and previously in Europe; however, 7 is not approved in the US. Metabolized by CYP2C9 (major) and CYP2C19 (minor), idiosyncratic hepatotoxic events associated with 7 are hypothesized to result from CYP biotransformation down-stream from initial metabolite 8 (McDonald and Bettie, 2007). In 2008, Lee and coworkers compared oral in vivo efficacy and concluded that 7 (100 mg/kg) produces a greater physiological effect (i.e. lowers urate level) than 4 (300 mg/day) or 5 (1000 mg/day) (Lee et al, 2008). Previous results from our laboratories illustrated 7 and 8 to have potent hURAT1 inhibitor properties (Enomoto et al, 2002; Shin et al, 2011). Therefore we sought to prepare a series of novel compounds and probe the interactions and structural requirement(s) related to transporter proteins involved in urate inhibition.